Specific heat capacity (C) is the amount of heat required to raise the temperature of a unit mass by one degree. For water it is measured in kJ/(kg·K) [SI] or Btu/(lbm·°F) [Imperial], and governs sizing of heat exchangers, heaters, and cooling systems. The heat transfer equation is:

Q = ṁ × C × ΔT

where Q is heat flow (W or BTU/hr), ṁ is mass flow rate, and ΔT is the temperature difference across the heat exchanger or system boundary.

Isochoric specific heat (Cv) applies to a constant-volume closed system. All added heat goes into raising the internal energy of the fluid with no work done on the surroundings.

Isobaric specific heat (Cp) applies to a constant-pressure open system. Added heat raises internal energy and also does expansion work, so Cp > Cv. For practical liquid-water engineering — open pipes, heat exchangers, and boilers below saturation — Cp is the correct value to use.

Both values vary with temperature. Cp reaches a minimum near 35–40 °C and rises steeply above 280 °C as water approaches the critical point (374 °C). When calculating mass and volume flow in water heating systems at elevated temperatures, specific heat should be corrected for temperature using the table above.